How forces arise

-The two forces in an interaction pair are always equal and opposite and act on different objects

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How things start working

-To make a vehicle/person start moving it needs to push against the ground

-When it pushes on the ground the ground pushes back and it will start to move

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Friciton

- Friction is an unusual force

- It adjusts its size in response to the situation – up to a limit

- This limit depends on the objects and the surfaces involved

- The force of friction arises due to lots of tiny welds that have to be broken as an object slides against another

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Reaction of surfaces

-If an object is placed on a surface it squashes or distorts the surface

-The surface exerts a reaction force on the object

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Adding forces

-If there is a force acting on an object and it is not moving there must be another force balancing the first one

-If they balance we say the “resultant force” is zero

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Speed

-Average speed = distance / time

-Instantaneous speed – when average speed is measured over very short time intervals

-Speed cameras detect speeding cars

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Motion Graphs

-Distance – time graph: gradient/slope shows speed

-Speed – time graph: gradient shows acceleration

-Velocity – time graph: also shows direction of motion

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Force and change of momentum

-Momentum = mass x velocity

- Change of momentum caused by a force:

Change of momentum = force x time

(time is for how long the force acts)

-Conservation of momentum – in an interaction the total change in momentum is zero

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Car Safety

In a collision the force on passengers can be great. Cars are designed to reduce these forces:

-Crumple zones – increase the collision time

-Seat belts – stretch to make the change of momentum longer

-Air bags – cushion impact to reduce your momentum slowly

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Factors involved

-Collision time – the size of force on the car depends on the time the collision lasts

-Momentum – the bigger the time, the smaller the force

In summary, the longer it takes to reduce the passenger’s speed to zero, the smaller the force they experience.

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Laws of Motion

-Law 1 – if the resultant force acting on an object is zero, the momentum of the object does not change-Law 2 – if there is a resultant force acting on an object, the momentum will change (c.o.m.=r.fx time) and is in the same direction

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Motion

-Stationary objects have a resultant force that is zero

-Objects moving at a constant speed also have a resultant force that is zero

-Speeding up or slowing down- overall resultant force exists

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Work done

-When a force causes movement of an object, work is done

- Use the equation:

work done by a force = force × distance moved by the force

(joule, J) (newton, N) (metre, m)

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Change of energy

-The energy of a moving object is called kinetic energy

-As an object falls, its gravitational potential energy decreases

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Work and change of energy (cont.)

-Understand that when work is doneonan object, the energy of the object increases and

- When work is donebyan object, the energy of the object decreases according to the relationship:

change in energy = work done

(joule, J) (joule, J)

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From potential to kinetic energy

- When an object is lifted to a higher position above the ground, work is done by the lifting force against the gravitational force acting on the object (its weight);

-this increases the object’s gravitational potential energy (GPE);

-use the equation:

change in GPE = weight × vertical height difference

(joule, J) (newton, N) (metre, m)

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Changes in kinetic energy

-When work is done to make an object move faster the kinetic energy increase.

-Change in energy = work done

-So,

change in energy = force x distance

However, some work is wasted due to the force of friction.

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Conservation of energy

When an object falls it –

-Loses gravitational potential energy

-Gains kinetic energy

-If friction is small enough to ignore then

Amount of GPE lost = amount of KE gained

We use this formula to calculate KE:Gain in KE = ½ mass x velocity squared